Sounds like a frustrating regression. Before chalking it up to a bug, a few things worth trying:
The transaction cache in Xcode's StoreKit environment gets into weird states after major version upgrades. From Xcode, open Debug > StoreKit > Manage Transactions, delete everything in there, then clean build folder (Shift-Cmd-K), nuke DerivedData, and restart Xcode. On the running target, delete the app entirely before re-running.
Second, check your scheme's StoreKit configuration setting. The Xcode 16 to 26 migration has been resetting "StoreKit Configuration" to None on some projects. Edit Scheme > Run > Options, confirm your .storekit file is still selected.
Third, when purchases silently fail without even showing the confirmation sheet, it's usually the store failing to resolve the product ID. Open the .storekit file, hit the refresh/sync icon, and confirm every product identifier matches what your code requests exactly (it's case-sensitive).
Did macOS also update around the same time? When it's not Xcode, it's usually commercekit or storekitd on the host Mac. Console.app filtered by those processes while you try a purchase will tell you if the daemon is the actual culprit.